Network Basics and Concepts

By: Dr. Najmadin W. Boskany

Computer Networks

Basic Concepts

Before examining the specific of how data are transmitted from one device to another, it is important to understand the relationship between the communicating devices.

These general concepts provide the basis for this relationship:

Line Configuration (Type of Connections) Topology Transmission Modes Categories of Networks Internetworks

Line Configuration

Line Configuration

Line configuration refers to the way two or more communication devices attach to a link.

A link is the physical communication pathway that transfers data from one device to another.

There are two possible line configurations:

point-to-point and multipoint.

Point-to-point

A point-to-point line configuration provides a dedicated link between two devices.

The entire capacity of the channel is reserved for transmission between those two devices (i.e. change TV channel by infrared remote control).

Multipoint (multidrop)

A multipoint line configuration is one in which more than two specific devices share a single link.

In a multipoint environment, the capacity of channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is spatially shared line configuration. If users must take turns, it is a time shared line configuration.

Topology

Topology

The term Topology refers to the way a network is laid out, either Physically or logically (arrangement). Two or more devices connect to a link; two or more links form a topology.

The topology of a network is a geometric representation of a relationship of all links and linking devices (nodes) to each other.

A consideration when choosing a topology is the relative status of the devices to be linked( peer-to-peer, primary-Secondary).

Two relationships are possible: peer-to-peer, where devices share the link equally, and primary-secondary, where one device controls traffic and the others must transmit through it.

Topology

Four basic types: mesh, star, bus, and ring. Ring and Mesh topologies are more convenient

for peer-to-peer transmission. While Star is more convenient for primary-

secondary( master-slave). A Bus topology is equally convenient for either.

Mesh Topology

Dedicated point-to-point links to every other device.

Number of link (physical channel)=n(n-1)/2 to link n devices.

Number of input/output (I/O) ports per device =n-1

Advantage of Mesh Topology

Use of dedicated links guarantees that each connection can carry its data load, thus eliminating the traffic problems that can occur when links must be shard by multiple devices.

A mesh topology is robust (forceful). If one link becomes unusable, it does not incapacitate the entire system.

Privacy and Security: when every message sent travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to message.

point-to-point links make fault identification and fault isolation easy .This facility enables the network manager to discover the exact location of the fault and aids in finding its cause and solution.

Disadvantage of Mesh Topology Amount of cabling and the number of I/O ports are

required. Installation and reconfiguration are difficult. The sheer bulk (complete size) of the wiring can be

grater than the available space (in walls, ceilings, or floors) can accommodate (hold).

The hardware required to connect mesh topology is usually implemented in a limited fashion (i.e. as a backbone connecting the main computers of a hybrid network that can include several other topologies).

Star Topology Each device has a dedicated point-to-point links

only to central controller (hub or switch). The devices are not linked to each other directly.

Unlike mesh topology, star topology does not allow direct traffic between devices.

The controller acts as an exchange (center of activity). If one device wants to send data to another, it sends to the controller, which then relays the data to the other connected device.

Advantages of Star Topology

Less expensive than mesh topology In star each device needs only one link and one I/O

port to connect it to any number of other. This factor also makes it easy to install and reconfigure.

Far less cabling needs to be housed, and addition, moves, and deletions involve only one connection: between that device and the hub.

Robustness: if one link fails, only that link is affected. All other links remain active, this factor also lends itself to easy fault identification and fault isolation.

As long as the hub is working, it can be used to monitor link problems and bypass defective (faulty) links.

Disadvantages of Star Topology Star topology requires far less cable than mesh

topology, each node must be linked to a central hub. For this reason more cabling is required in a star than in some other topologies ( such as ring, or bus)

In case a hub is stopped, all devices will stop. Less security exist, than mesh topology.

Ring Topology

Each device has a dedicated point-to-point line configuration only with the two devices on either neighbor of it.

A signal is passed along the ring in one direction , from device to device until it reaches its destination.

Each device in the ring functions as a repeater, when a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

Advantages of Ring Topology

A ring is relatively easy to install and reconfigure. Each device is linked only to its immediate

neighbor. To add or delete a device requires moving only two

connections. The only constraints are media and traffic

considerations (maximum ring length and number of devices).

In addition, fault isolation is simplified generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period the alarm alerts the network operator to the problem and its location.

Disadvantages of Ring Topology However, unidirectional traffic can be a

disadvantage.

In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using dual ring or a switch capable of closing off the break.

Bus Topology

The preceding examples all describe point-to-point configurations. A Bus topology, in the other hand, is multipoint configuration.

One long cable acts as a backbone to link all the devices in the network. Nodes are connected to the Bus cable by drop lines and taps.

A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into main cable or punctures the sheathing of a cable to create a contact with the metallic core.

Bus Topology

As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker the farther it has to travel. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps.

Advantages of Bus Topology

Ease of installation. Backbone cable can be laid along the most

efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh, star topologies.

In a star for example, four network devices in the same room require four lengths of cable reaching all the way to the hub. In a Bus, the redundancy is eliminated.

Only the backbone cable stretches through the entire facility.

Disadvantages of Bus Topology Difficult reconfiguration and fault isolation. A bus usually designed to be optimally efficient at

installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in

quality. This degradation can be controlled by limiting the

number and spacing of devices connected to a given length of cable.

Adding new devices may therefore require modification or replacement of the backbone.

In addition, fault or break in the bus cable stops all transmission, even between devices on the same side of the problem.

The damaged area reflects signals back in the direction of origin, creating noise in both directions.

Hybrid Topology

Often a network combines several topologies as subnetworks linked together in a larger topology. For instance, one department of a business may have decided to use a bus topology while another department has a ring.

The two can be connected to each other via central controller in a star topology.

Transmission Modes

Transmission Modes

The term transmission mode is used to define the direction of signal (information) flow between two linked devices.

There are three types of transmission modes:

simplex, half-duplex, and full-duplex.

Direction of Data Flow ?

Simplex

In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two stations on a link can transmit; the other can only receive.

Keyboards and traditional monitors are both examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output.

Half-Duplex

In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice versa.

The half-duplex mode is like a one-lane road with two-directional traffic. While cars are traveling one direction, cars going the other way must wait.

each can transmit/receive; communication must alternate.

• In half-duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time.

• Walkie-talkies device is best example on half-duplex mode.

Half-Duplex

Full-Duplex

In Full-Duplex mode both station can transmit and receive simultaneously.

The full-duplex mode is like two-way street with traffic flowing in both directions at the same time.

In full-duplex mode, signal going in either direction share the capacity of the link.

This sharing can occur in two ways: either the link must contain two physically separate transmission paths, one for sending and the other for receiving, or the capacity of the channel is divided between signals traveling in opposite directions.

Full-Duplex

Network Categories

Network Categories

We are generally referring to three primary categories:

Local Area Network (LANs), Metropolitan Area Network (MANs) Wide Area Network (WANs)

Network Categories

Based on size, ownership, distance covered, and physical architecture

Local Area Network (LAN) – small geographical area.

Metropolitan Area Network (MANs) – network extended over an entire city.

Wide Area Network (WAN) – large geographical area.

Network Categories

Also some other categories are exists:

Tiny Area Networks (TANs) Peronal Area Networks (PANs) Campus Area Networks (CANs) Distributed Area Networks (DANs) Storage Area Networks (SANs)

LAN

A local area network is usually privately owned and links the devices in a single office, building, campus. Depending on the needs of any organization and the type of technology used.

A LAN can be as simple as two PCs and a printer in some one’s home office, or it can extend throughout a company and include voice, sound, and video peripheral.

Currently LAN size is limited to few kilometers. LANs are designed to allow resources to be

shared between PC and workstations.

LAN

The resources to be shared can be hardware (printer) and software (application program) or data base.

In general, a given LAN will use only one type of transmission medium.

The most common LAN topologies are Bus, Ring, and Star.

Traditionally, LANs have data rates in the 4 to 16 Mbps range. Today, however, speeds are increasing, can reach 100Mbps (Fast Ethernet) and 1000Mbps (Giga Ethernet).

LAN

MAN

A metropolitan area network is designed to extend over an entire city. It may be single network such as cable television network, or it may be a means of connecting a number of LANs into a larger network so that resources may be shared LAN-to-LAN as well as device-to-device. For example, a company can use a MAN to connect the LANs in all of its offices throughout a city.

A MAN may be wholly owned and operated by a private company, or it may be a service provided by a public company, such as a local telephone company.

MAN

WAN

A Wide Area Network provides long-distance transmission of data, voice, image, and video information over large geographical areas that may comprise a country, a continent, or even a whole world.

In contrast to LANs (which depend on their own hardware for transmission), WANs may utilize public, leased, or private communication devices, usually in combinations, and can therefore distance an unlimited number of miles.

A WAN that is wholly owned and used by a single company is often referred to as an enterprise network.

WAN

Internetworks

internetworks

Connection of two or more networks called (internetwork or internet).

Internet vs. internet (notice case).

internet (lowercase i) should not be confused with the Internet (uppercase I). The first is a generic term used to mean an interconnection of networks. The second is the name of a specific worldwide network.

internetwork (internet)

R

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R

R

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WAN

MANLAN

LAN

LAN

LAN

LAN

The Internet

Collaboration of more than hundreds of thousands of interconnected networks.

1969 – started as ARPAnet, a small network of connected computers.

1972 - Cerf and Khan – packet delivery and Transmission Control Protocol (TCP)

Shortly thereafter – evolution of TCP/IP